Remove some timestamp format assumptions

Remove some timestamp format assumptions

Don’t assume that current-time and plain encode-time return
timestamps in (HI LO US PS) format.
* lisp/gnus/gnus-art.el (article-make-date-line)
(article-lapsed-string):
* lisp/gnus/gnus-demon.el (gnus-demon-time-to-step):
* lisp/gnus/gnus-diary.el (gnus-user-format-function-d):
* lisp/gnus/nnmaildir.el (nnmaildir-request-expire-articles):
* lisp/net/pop3.el (pop3-uidl-dele):
* lisp/org/ox-publish.el (org-publish-sitemap):
* lisp/vc/vc-hg.el (vc-hg-state-fast):
Simplify and remove assumptions about timestamp format.
* lisp/gnus/gnus-art.el (article-lapsed-string):
* lisp/gnus/gnus-diary.el (gnus-user-format-function-d):
Do not worry about time-subtract returning nil; that's not possible.
* lisp/gnus/gnus-diary.el (gnus-user-format-function-d):
Avoid race due to duplicate current-time calls.
* lisp/vc/vc-hg.el (vc-hg--time-to-integer): Remove; no longer used.